Over 1.5 million
living animal
species have been
described, of which around 1.05 million are
insects, over 85,000 are
molluscs, and around 65,000 are
vertebrates. It has been estimated there are as many as 7.77 million animal species on Earth. Animal body lengths range from 8.5 μm (0.00033 in) to 33.6 m (110 ft). They have complex
ecologies and
interactions with each other and their environments, forming intricate
food webs. The scientific study of animals is known as
zoology, and the study of animal behaviors is known as
ethology.
Animals first appear in the fossil record in the late
Cryogenian period, and diversified in the subsequent
Ediacaran. Earlier evidence of animals is still controversial; the
sponge-like organism Otavia has been dated back to the
Tonian period at the start of the
Neoproterozoic, but its identity as an animal is heavily contested.[5] Nearly all modern animal phyla became clearly established in the fossil record as
marine species during the
Cambrian explosion, which began around 539
million years ago (Mya), and most
classes during the
Ordovician radiation 485.4 Mya. 6,331 groups of
genes common to all living animals have been identified; these may have arisen from a single
common ancestor that lived about 650 Mya during the
Cryogenian period.
The word animal comes from the Latin noun animal of the same meaning, which is itself derived from Latin animalis 'having breath or soul'.[6] The biological definition includes all members of the kingdom Animalia.[7] In colloquial usage, the term animal is often used to refer only to nonhuman animals.[8][9][10][11] The term metazoa is derived from Ancient Greek μετα (meta) 'after' (in biology, the prefix meta- stands for 'later') and ζῷᾰ (zōia) 'animals', plural of ζῷον zōion 'animal'.[12][13]
All animals are composed of cells, surrounded by a characteristic
extracellular matrix composed of
collagen and elastic
glycoproteins.[23] During development, the animal extracellular matrix forms a relatively flexible framework upon which cells can move about and be reorganised, making the formation of complex structures possible. This may be calcified, forming structures such as
shells,
bones, and
spicules.[24] In contrast, the cells of other multicellular organisms (primarily algae, plants, and
fungi) are held in place by cell walls, and so develop by progressive growth.[25] Animal cells uniquely possess the
cell junctions called
tight junctions,
gap junctions, and
desmosomes.[26]
With few exceptions—in particular, the sponges and
placozoans—animal bodies are differentiated into
tissues.[27] These include
muscles, which enable locomotion, and
nerve tissues, which transmit signals and coordinate the body. Typically, there is also an internal
digestive chamber with either one opening (in Ctenophora, Cnidaria, and flatworms) or two openings (in most bilaterians).[28]
Nearly all animals make use of some form of sexual reproduction.[29] They produce
haploidgametes by
meiosis; the smaller, motile gametes are
spermatozoa and the larger, non-motile gametes are
ova.[30] These fuse to form
zygotes,[31] which develop via
mitosis into a hollow sphere, called a blastula. In sponges, blastula larvae swim to a new location, attach to the seabed, and develop into a new sponge.[32] In most other groups, the blastula undergoes more complicated rearrangement.[33] It first
invaginates to form a
gastrula with a digestive chamber and two separate
germ layers, an external
ectoderm and an internal
endoderm.[34] In most cases, a third germ layer, the
mesoderm, also develops between them.[35] These germ layers then differentiate to form tissues and organs.[36]
Animals evolved in the sea. Lineages of arthropods colonised land around the same time as
land plants, probably between 510 and 471 million years ago during the
Late Cambrian or Early
Ordovician.[55]Vertebrates such as the
lobe-finned fishTiktaalik started to move on to land in the late
Devonian, about 375 million years ago.[56][57] Animals occupy virtually all of earth's
habitats and microhabitats, with
faunas adapted to salt water, hydrothermal vents, fresh water, hot springs, swamps, forests, pastures, deserts, air, and the interiors of other organisms.[58] Animals are however not particularly
heat tolerant; very few of them can survive at constant temperatures above 50 °C (122 °F)[59] or in the most extreme cold deserts of continental
Antarctica.[60]
The
blue whale (Balaenoptera musculus) is the largest animal that has ever lived, weighing up to 190
tonnes and measuring up to 33.6 metres (110 ft) long.[61][62][63] The largest extant terrestrial animal is the
African bush elephant (Loxodonta africana), weighing up to 12.25 tonnes[61] and measuring up to 10.67 metres (35.0 ft) long.[61] The largest terrestrial animals that ever lived were
titanosaursauropod dinosaurs such as Argentinosaurus, which may have weighed as much as 73 tonnes, and Supersaurus which may have reached 39 meters.[64][65] Several animals are microscopic; some
Myxozoa (
obligate parasites within the Cnidaria) never grow larger than 20
μm,[66] and one of the smallest species (Myxobolus shekel) is no more than 8.5 μm when fully grown.[67]
Numbers and habitats of major phyla
The following table lists estimated numbers of described extant species for the major animal phyla,[68] along with their principal habitats (terrestrial, fresh water,[69] and marine),[70] and free-living or parasitic ways of life.[71] Species estimates shown here are based on numbers described scientifically; much larger estimates have been calculated based on various means of prediction, and these can vary wildly. For instance, around 25,000–27,000 species of nematodes have been described, while published estimates of the total number of nematode species include 10,000–20,000; 500,000; 10 million; and 100 million.[72] Using patterns within the
taxonomic hierarchy, the total number of animal species—including those not yet described—was calculated to be about 7.77 million in 2011.[73][74][b]
Evidence of animals is found as long ago as the
Cryogenian period.
24-Isopropylcholestane (24-ipc) has been found in rocks from roughly 650 million years ago; it is only produced by sponges and
pelagophyte algae. Its likely origin is from sponges based on
molecular clock estimates for the origin of 24-ipc production in both groups. Analyses of pelagophyte algae consistently recover a
Phanerozoic origin, while analyses of sponges recover a
Neoproterozoic origin, consistent with the appearance of 24-ipc in the fossil record.[89][90]
The first body fossils of animals appear in the
Ediacaran, represented by forms such as Charnia and Spriggina. It had long been doubted whether these fossils truly represented animals,[91][92][93] but the discovery of the animal lipid
cholesterol in fossils of Dickinsonia establishes their nature.[94] Animals are thought to have originated under low-oxygen conditions, suggesting that they were capable of living entirely by
anaerobic respiration, but as they became specialized for aerobic metabolism they became fully dependent on oxygen in their environments.[95]
Some palaeontologists have suggested that animals appeared much earlier than the Cambrian explosion, possibly as early as 1 billion years ago.[102] Early fossils that might represent animals appear for example in the 665-million-year-old rocks of the
Trezona Formation of
South Australia. These fossils are interpreted as most probably being early
sponges.[103]Trace fossils such as tracks and burrows found in the
Tonian period (from 1 gya) may indicate the presence of
triploblastic worm-like animals, roughly as large (about 5 mm wide) and complex as earthworms.[104] However, similar tracks are produced by the giant single-celled protist Gromia sphaerica, so the Tonian trace fossils may not indicate early animal evolution.[105][106] Around the same time, the layered mats of
microorganisms called
stromatolites decreased in diversity, perhaps due to grazing by newly evolved animals.[107] Objects such as sediment-filled tubes that resemble trace fossils of the burrows of wormlike animals have been found in 1.2 gya rocks in North America, in 1.5 gya rocks in Australia and North America, and in 1.7 gya rocks in Australia. Their interpretation as having an animal origin is disputed, as they might be water-escape or other structures.[108][109]
Ros-Rocher and colleagues (2021) trace the origins of animals to unicellular ancestors, providing the external phylogeny shown in the cladogram. Uncertainty of relationships is indicated with dashed lines.[116]
Hox genes are found in the Placozoa,[119][120] Cnidaria,[121] and Bilateria.[122][123] 6,331 groups of
genes common to all living animals have been identified; these may have arisen from a single
common ancestor that lived
650 million years ago in the
Precambrian. 25 of these are novel core gene groups, found only in animals; of those, 8 are for essential components of the
Wnt and
TGF-beta signalling pathways which may have enabled animals to become multicellular by providing a pattern for the body's system of axes (in three dimensions), and another 7 are for
transcription factors including
homeodomain proteins involved in the
control of development.[124][125]
Giribet and Edgecombe (2020) provide what they consider to be a consensus internal phylogeny of the animals, embodying uncertainty about the structure at the base of the tree (dashed lines).[126]
An alternative phylogeny, from Kapli and colleagues (2021), proposes a clade
Xenambulacraria for the Xenacoelamorpha + Ambulacraria; this is either within Deuterostomia, as sister to Chordata, or the Deuterostomia are recovered as paraphyletic, and Xenambulacraria is sister to the proposed clade
Centroneuralia, consisting of Chordata + Protostomia.[127]
Sponges are physically very distinct from other animals, and were long thought to have diverged first, representing the oldest animal phylum and forming a
sister clade to all other animals.[128] Despite their morphological dissimilarity with all other animals, genetic evidence suggests sponges may be more closely related to other animals than the comb jellies are.[129][130] Sponges lack the complex organization found in most other animal phyla;[131] their cells are differentiated, but in most cases not organised into distinct tissues, unlike all other animals.[132] They typically feed by drawing in water through pores, filtering out small particles of food.[133]
The comb jellies and Cnidaria are radially symmetric and have digestive chambers with a single opening, which serves as both mouth and anus.[134] Animals in both phyla have distinct tissues, but these are not organised into discrete
organs.[135] They are
diploblastic, having only two main germ layers, ectoderm and endoderm.[136]
The tiny placozoans have no permanent digestive chamber and no symmetry; they superficially resemble amoebae.[137][138] Their phylogeny is poorly defined, and under active research.[129][139]
The remaining animals, the great majority—comprising some 29 phyla and over a million species—form a
clade, the Bilateria, which have a bilaterally symmetric
body plan. The Bilateria are
triploblastic, with three well-developed germ layers, and their tissues
form distinct organs. The digestive chamber has two openings, a mouth and an anus, and there is an internal body cavity, a
coelom or pseudocoelom. These animals have a head end (anterior) and a tail end (posterior), a back (dorsal) surface and a belly (ventral) surface, and a left and a right side.[140][141]
Having a front end means that this part of the body encounters stimuli, such as food, favouring
cephalisation, the development of a head with
sense organs and a mouth. Many bilaterians have a combination of circular
muscles that constrict the body, making it longer, and an opposing set of longitudinal muscles, that shorten the body;[141] these enable soft-bodied animals with a
hydrostatic skeleton to move by
peristalsis.[142] They also have a gut that extends through the basically cylindrical body from mouth to anus. Many bilaterian phyla have primary
larvae which swim with
cilia and have an apical organ containing sensory cells. However, over evolutionary time, descendant spaces have evolved which have lost one or more of each of these characteristics. For example, adult echinoderms are radially symmetric (unlike their larvae), while some
parasitic worms have extremely simplified body structures.[140][141]
Genetic studies have considerably changed zoologists' understanding of the relationships within the Bilateria. Most appear to belong to two major lineages, the
protostomes and the
deuterostomes.[143] It is often suggested that the basalmost bilaterians are the
Xenacoelomorpha, with all other bilaterians belonging to the subclade
Nephrozoa[144][145][146] However, this suggestion has been contested, with other studies finding that xenacoelomorphs are more closely related to Ambulacraria than to other bilaterians.[127]
Protostomes and deuterostomes differ in several ways. Early in development, deuterostome embryos undergo radial
cleavage during cell division, while many protostomes (the
Spiralia) undergo spiral cleavage.[147]
Animals from both groups possess a complete digestive tract, but in protostomes the first opening of the
embryonic gut develops into the mouth, and the anus forms secondarily. In deuterostomes, the anus forms first while the mouth develops secondarily.[148][149] Most protostomes have
schizocoelous development, where cells simply fill in the interior of the gastrula to form the mesoderm. In deuterostomes, the mesoderm forms by
enterocoelic pouching, through invagination of the endoderm.[150]
The Ecdysozoa are protostomes, named after their shared
trait of
ecdysis, growth by moulting.[157] They include the largest animal phylum, the
Arthropoda, which contains insects, spiders, crabs, and their kin. All of these have a body divided into
repeating segments, typically with paired appendages. Two smaller phyla, the
Onychophora and
Tardigrada, are close relatives of the arthropods and share these traits. The ecdysozoans also include the Nematoda or roundworms, perhaps the second largest animal phylum. Roundworms are typically microscopic, and occur in nearly every environment where there is water;[158] some are important parasites.[159] Smaller phyla related to them are the
Nematomorpha or horsehair worms, and the
Kinorhyncha,
Priapulida, and
Loricifera. These groups have a reduced coelom, called a pseudocoelom.[160]
The Spiralia are a large group of protostomes that develop by spiral cleavage in the early embryo.[161] The Spiralia's phylogeny has been disputed, but it contains a large clade, the superphylum
Lophotrochozoa, and smaller groups of phyla such as the
Rouphozoa which includes the
gastrotrichs and the
flatworms. All of these are grouped as the
Platytrochozoa, which has a sister group, the
Gnathifera, which includes the
rotifers.[162][163]
In the
classical era, Aristotle
divided animals,[e] based on his own observations, into those with blood (roughly, the vertebrates) and those without. The animals were then
arranged on a scale from man (with blood, 2 legs, rational soul) down through the live-bearing tetrapods (with blood, 4 legs, sensitive soul) and other groups such as crustaceans (no blood, many legs, sensitive soul) down to spontaneously generating creatures like sponges (no blood, no legs, vegetable soul).
Aristotle was uncertain whether sponges were animals, which in his system ought to have sensation, appetite, and locomotion, or plants, which did not: he knew that sponges could sense touch, and would contract if about to be pulled off their rocks, but that they were rooted like plants and never moved about.[169]
In 1758,
Carl Linnaeus created the first
hierarchical classification in his Systema Naturae.[170] In his original scheme, the animals were one of three kingdoms, divided into the classes of
Vermes,
Insecta,
Pisces,
Amphibia,
Aves, and
Mammalia. Since then the last four have all been subsumed into a single phylum, the
Chordata, while his Insecta (which included the crustaceans and arachnids) and Vermes have been renamed or broken up. The process was begun in 1793 by
Jean-Baptiste de Lamarck, who called the Vermes une espèce de chaos (a chaotic mess)[f] and split the group into three new phyla: worms, echinoderms, and polyps (which contained corals and jellyfish). By 1809, in his Philosophie Zoologique, Lamarck had created 9 phyla apart from vertebrates (where he still had 4 phyla: mammals, birds, reptiles, and fish) and molluscs, namely
cirripedes, annelids, crustaceans, arachnids, insects, worms,
radiates, polyps, and
infusorians.[168]
In his 1817 Le Règne Animal,
Georges Cuvier used
comparative anatomy to group the animals into four embranchements ("branches" with different body plans, roughly corresponding to phyla), namely vertebrates, molluscs, articulated animals (arthropods and annelids), and
zoophytes (radiata) (echinoderms, cnidaria and other forms).[172] This division into four was followed by the embryologist
Karl Ernst von Baer in 1828, the zoologist
Louis Agassiz in 1857, and the comparative anatomist
Richard Owen in 1860.[173]
In 1874,
Ernst Haeckel divided the animal kingdom into two subkingdoms: Metazoa (multicellular animals, with five phyla: coelenterates, echinoderms, articulates, molluscs, and vertebrates) and Protozoa (single-celled animals), including a sixth animal phylum, sponges.[174][173] The protozoa were later moved to the former kingdom
Protista, leaving only the Metazoa as a synonym of Animalia.[175]
The human population exploits a large number of other animal species for food, both of
domesticated livestock species in
animal husbandry and, mainly at sea, by hunting wild species.[176][177] Marine fish of many species are
caught commercially for food. A smaller number of species are
farmed commercially.[176][178][179] Humans and their
livestock make up more than 90% of the biomass of all terrestrial vertebrates, and almost as much as all insects combined.[180]
Invertebrates including
cephalopods,
crustaceans, and
bivalve or
gastropod molluscs are hunted or farmed for food.[181]Chickens,
cattle,
sheep,
pigs, and other animals are raised as livestock for meat across the world.[177][182][183] Animal fibres such as wool are used to make textiles, while animal
sinews have been used as lashings and bindings, and leather is widely used to make shoes and other items. Animals have been hunted and farmed for their fur to make items such as coats and hats.[184] Dyestuffs including
carmine (
cochineal),[185][186]shellac,[187][188] and
kermes[189][190] have been made from the bodies of insects.
Working animals including cattle and horses have been used for work and transport from the first days of agriculture.[191]
^The application of
DNA barcoding to taxonomy further complicates this; a 2016 barcoding analysis estimated a total count of nearly 100,000
insect species for
Canada alone, and extrapolated that the global insect fauna must be in excess of 10 million species, of which nearly 2 million are in a single fly family known as gall midges (
Cecidomyiidae).[75]
^The French prefix une espèce de is pejorative.[171]
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